It is well known that fine particles of crosslinked urea-formaldehyde polymer having a diameter of 0.1 to 1.0.mu. are useful as the so-called filler which is added to paper for the purpose of improving certain properties (such as opacity and the like) thereof. One process for producing crosslinked urea-formaldehyde polymer particles is disclosed in Japanese Patent Laid-Open No. 135893/'79. According to this process, a precondensate solution is prepared by mixing an aqueous formaldehyde solution, urea, sodium carboxymethyl cellulose and water, adjusting the resulting mixture to pH 7 with an aqueous solution of sodium hydroxide, and then allowing it to react. Using an in-line mixer, this precondensate solution is continuously mixed with an aqueous solution of sulfuric acid having a concentration of several percent by weight. Before commencement of its solidification, the resulting mixture is fed onto a moving endless belt made of acid resistant rubber and allowed to solidify (gel) thereon. After the addition of water to the solidified gel-like material thus obtained, the resulting mixture is stirred to form a slurry. This slurry is neutralized with an aqueous alkaline solution and then ground in a colloid mill to obtain a dispersion of crosslinked urea-formaldehyde polymer particles (hereinafter referred to as a filler dispersion).
The filler dispersion thus obtained usually contains 90 to 95% by weight of water, thus requiring great expenses for transportation and storage. Accordingly, it has been common practice to dehydrate the filler dispersion by means of a filtering device and thereby separate it into cake and filtrate which is then discarded.
However, since the filtrate contains organic materials such as unreacted formaldehyde and urea, direct discharge thereof may cause environmental pollution. For this reason, the filtrate has been discharged after being disposed of by biodegradation and the like. Since this waste water disposal is very expensive, it would be desirable to establish a process in which the filtrate is effectively utilized instead of being discharged out of the system as waste water.
The present inventors have made repeated studies on this subject and have found that, in order to utilize the filtrate effectively, it must be reused at a fixed stage of the reaction. And yet, even if the filtrate is reused at a fixed stage of the reaction, the resulting crosslinked urea-formaldehyde polymer particles may occasionally contain a considerable amount of particles having a diameter of greater than 1.mu.. It has been found that such a product fails to produce a satisfactory improvement in certain properties (such as opacity and the like) of paper. As a result of further studies on this point, the present inventors have found that crosslinked urea-formaldehyde polymer particles having a diameter of 0.1 to 1.0.mu. can be obtained only by reusing the filtrate at a fixed stage of the reaction and, at the same time, using a specified type of precondensate. The present invention has been completed on the basis by these discoveries.